Dogs and diet

Diet is important

Diet is very important because it affects dogs' health and behaviour. It's also a vast area of study, with many intense debates, such as what owners should look for in commercial foods, and whether home-prepared or processed foods are better. We offer some answers to common queries here, with suggestions for further reading at the end of this article, which will be especially helpful if you want to use home-cooked food.

It's worth checking with your vet before making alterations to your dog's diet, especially if you want to give supplements, or if your dog has particular breed-related dietary needs, or special needs due to illness. You need to get the dosage right if you give supplements or you risk giving your dog an overdose of some minerals or vitamins. Dogs may also have more than one medical condition, and you may need help to work out which takes precedence when making decisions on diet. Vets may not know a great deal about canine nutrition, unless they have developed a particular interest in the topic, but they do know about the common diet-related complaints that they see at their surgeries. The best vets are prepared to say when they can't answer your questions immediately, and will check out topics for you.

Sudden major changes in dogs' diets can cause digestive upsets, so make radical changes gradually, mixing some of the old food with the new. It's especially important to introduce new foods to puppies gradually. Pups can develop allergies if their diets change suddenly, so it's safer to try one food at a time and a little at a time.

What to look for in a commercial dog food: Nutrients, ingredients and pet food labels

You're faced with an array of dog foods at a store. How can you decide which to choose? First, check the labels and see what they tell you. You should find a list of both nutrients and ingredients, together with recommended servings. This won't tell you all you need to know, but it's a start!

Dogs need to have the right balance of nutrients, of proteins, fats, carbohydrates, vitamins and minerals in line with ‘good practice’ in canine nutrition. However, the nutrients also have to be accessible to the dog. Dogs aren’t able to digest some foods well, for example, grain isn’t the best source of protein for dogs, and doesn’t provide all the amino acids they need. Many dogs react to specific foods like maize and chicken, and of course they’ll be unable to benefit from nutrients in these foods if the foods make them sick or squitty. So, the nutrients levels listed on pack and can labels don't tell you much on their own, you also need to check the ingredients.

Meat and fish are good sources of protein for dogs, so choose from foods that have meat or fish as the main ingredient, which is listed first on pack and can labels. Other ingredients, such as cereals and vegetable derivatives may, however, add up to more than the meat and/or fish content, even if meat or fish head the list, and it's worth checking the actual proportions with manufacturers.

Is everything described as 'meat' in pet foods good for dogs? The ingredients lists on can and pack labels can be very vague, and may just describe meat as 'meat and animal derivatives', without saying which animal the meat came from. This can be a problem if your dog is allergic to certain types of meat. Other labels may give information on which animals or birds the meat comes from, but don't specify which body parts of animals are used. This could be, for example, beaks and feathers, or smooth intestine. The important issue should be what is good for dogs, rather than whether the source of food revolts you as a human. Wolves eat rabbits, skin, bone, stomach contents and all, and that is what they are designed to do, however revolting humans might find it. However, some things owners should worry about include drug residues in slaughtered animals used for pet food, or toxins resulting from E coli infections from long-dead animals. One concern about commercial foods is that pet owners have little way of knowing about the quality of ingredients used.

There’s some evidence that oily fish provides protection against cancer, heart disease, arthritis, and skin disease. However, again labels give little information on what types of fish, and parts of the fish are used in dog foods containing fish.

Pet food labels may give information on vitamins and minerals, but do not always say what form they’re in, which is important because some forms are more bioavailable - accessible to the dog - than others. There are disagreements on what constitutes the correct levels of vitamins and minerals, but canine nutritionists agree that it is possible to give dogs an overdose. The ratios may also be important, eg ratios of calcium to phosphorous, or essential fatty acids to vitamin E, and these may be difficult to calculate from information given on labels.

Are additives important?

‘Additives’ include preservatives, colourings, and flavour enhancers. Additives aren't always bad, and ‘natural’ doesn’t necessarily mean ‘good’. Deadly nightshade is ‘natural’ and can kill humans, but many preservatives and colourings haven’t been around for long, so their long-term effects are unknown. There are also additives which humans can tolerate but which dogs can’t, so it's not necessarily safe to feed dogs with human food scraps that contain additives, like bits of bacon, just because you can eat them and they don't seem to harm you.

Common sense says it's best to avoid highly coloured food. Your dog doesn't care what colour his food is, and is quite happy if it’s sludge brown. Smell, taste and texture are more important for dogs. Colourings are put there for humans. Avoiding highly coloured food means that you don't expose your dog to unnecessary risk. Flavour enhancers should also be avoided, not only because they may be harmful in themselves, but they may also incline dogs towards obesity.

Preservatives tend to be used in dry rather than canned commercial food, since canning is itself a way of preserving food. Once canned food has been opened it should be used on the same day, and any food left over from one meal stored in a cool place or in the fridge, with a cover. There’s some disagreement on whether dog-owners should avoid preservatives. Some preservatives, such as BHA may be carcinogenic, but evidence is mixed. Some foods have vitamin E added as a preservative, and this may be safer than BHA, but is also less effective as a preservative, Dry commercial food without preservatives will deteriorate fast.

Whatever commercial food you buy, it’s sensible to store it properly, sealed and in a cool, dry place, and use it before the ‘consume by’ date. Check that the food you buy has some time to go before its 'consume by' date expires. Owners often buy in bulk, to save money, but it's no saving if you end up paying vet bills to deal with skin problems and other conditions from feeding your dog on dry food that has lost nutritional value. Think of buying smaller amounts at a time, especially if you use ‘preservative-free’ food. It’s essential to use a large container with a lid if there’s any risk of kibble being infested with mice or other vermin. Vermin can spread disease, and infect you as well as your dog. Kibble that has gone mouldy through damp can also make dogs ill, even if the kibble has subsequently dried.

Pet food manufacturers themselves may refrain from using additives, but it’s, unclear whether the ingredients they use already contain additives. The resulting product may thus contain additives, though none has been added in the final manufacturing process. A claim that a product is 'additive free' is then no guarantee of quality, and some additives may be harmless, or even beneficial, but it's a start, since it's more likely that there are no unnecessary colourings and the like. There may also be links between skin allergies and behavioural problems and certain additives, which is one reason why home-cooked foods can be safer.

Dry or wet food?

Dry food is cheaper and more convenient. However, it may also contain more grain and preservatives, because grain is easier to store dry than meat, and canning is a way of preserving food.

Dogs need to drink more water if they’re fed dry food, and are more at risk from dehydration if they’re fed dried food and don’t have access to water. This is important since some owners remove water when they’re out of the house or at night, in the belief that this will help with house-training. Dogs are more likely to develop kidney and bladder problems if they’re fed dried food and deprived of water. If you have to leave your dog very long periods, he will need water and someone to come in and let him out for a wee. If you can't get help with dog care, then at least give him access to water and a place where he can wee, like a secure dog run with a roof, that he can get to through a dog door, and can't dig his way out of.

Some owners believe that crunching on dry food is better for dogs' teeth, but most dogs fed canned foods are also fed a crunchy mixer. There are better ways of protecting dogs teeth, such as giving them bones to chew on - see 'Does diet affect dogs' teeth and gums?' below.

What about extruded food?

Some dried extruded foods will swell up like a sponge if put in water - test your dog's food to see if it does this. Some owners fear that this type of food can cause problems by swelling up inside the dog. Diabetic dogs shouldn’t be fed extruded food, which may contain too high levels of simple carbohydrates.

How do you compare commercial pet foods?

You need to be able to do some basic maths if you are comparing wet and dry foods, to be able to compare like with like. First you need to remove moisture from the equation. You can do this by subtracting the moisture content from 100, which, for example, gives 25 for a food that is 75% moisture. Then divide the crude nutrient levels by 25 and multiply by 100 to give the dry-matter content percentage. So a food that has 8% protein levels and 75% moisture has a protein level of 32%, excluding moisture. It's also important to assess amounts per serving - a food may appear to have more of a vitamin if compared by dry weight, but if the servings are smaller, then the amount of the vitamin per serving will be reduced.

How can you obtain information from pet food companies?

Clearly, people whose dogs have special needs don't have enough information on pack and can labels on which to base an informed decision about which brand to buy. Even people whose dogs seem to thrive on just about any food may be worried about the lack of information on packs and cans. UK packs may, for example, may state that preservatives are used, but not specify which preservatives. It is worth asking the companies for more information. They may not answer your phone calls, letters and emails, but they’ll know that owners do care about what goes into their dogs' food. You will also be able to sort out which companies care about their reputations from the responses, or lack of responses you get.

Some owners believe that expensive foods are better foods, but this isn’t necessarily the case. The cost of pet foods may just reflect the marketing budget, rather than the quality of ingredients, and the research carried out into selecting ingredients. However, company that takes the trouble to answer your queries will care more about consumers that those which ignore you.

One question to ask is whether ingredients vary according to what is cheapest on the market, or whether the same ingredients are always used. This is especially important for dogs which can’t tolerate certain foods - you need to make sure that manufacturers can guarantee that their products will not contain those foods. You can also ask about feeding trials, and the breeds of dogs used, their ages, how long the trials lasted, and whether companies have information on the long-term effects of using their products.

Should dogs always stick to the same food?

There’s a strong case for feeding adult dogs without special needs a small range of commercial foods by rotation, rather than feeding any one commercial food exclusively throughout a dog's life. New discoveries are regularly being made about canine nutrition, and it’s possible that a food may be complete according to current accepted wisdom, but still lack important nutrients. If you vary the commercial foods you feed your dog, there is more chance that the dog will be able to find whatever may be lacking elsewhere. Dogs are also creatures of habit, and may dislike novel foods, so if they only eat one brand, owners may have problems if that brand becomes unavailable. Puppies need to have new foods introduced gradually, but once they’ve got used to a food this can be included in the weekly menu.

Is home-cooked food better?

There's no simple answer to this question. Kibble should meet basic nutritional requirements, while some people who feed home-cooked food fail to provide a balanced diet, or risk overdosing their dogs on supplements, or feed their dogs inappropriate human foods. Supporters of home-prepared foods argue that widespread usage of commercial food has led to an increase in a number of conditions, such as allergies and cancers. However, cancers may be more common these days simply because dogs live longer, or because people are more likely to keep pedigree breeds, and some breeds have particular vulnerabilities to cancer.

It’s certainly true that there’s a greater risk of making mistakes that damage your dog’s health if you decide on a home-prepared diet, for example, meat alone doesn’t provide enough calcium for dogs. Just because someone has a diet website doesn’t mean that they’re offering balanced diets! Vets who are also nutritionists are most likely to offer balanced diets. If you use recipes you find online, check the qualifications of the person offering them.

The best home-cooked food can be better than commercial food, so long as owners follow the guidelines of nutritionists, rather than following fads, or feeding dogs what they themselves like to eat. Each meal can be freshly prepared, and frozen if not used immediately. You have more control over what ingredients go into meals, which is especially important for dogs with food allergies and diabetes, and for geriatric dogs, which need easily-digestible food. Breeders sometimes worry about the effects of soya on bitches, since large amounts of soya can have a similar effect to oestrogen, so breeders sometimes home-cook to ensure they exclude soya. You also have more control over the quality of the ingredients when you home-cook, for example ensuring meat is fresh before it’s cooked.

The main difference between commercial and home-prepared foods is that commercial foods are prepared in bulk to be stored and sold, whereas home-prepared foods are made in smaller amounts and tend to be eaten fresh. Many owners have to use commercial foods due to lack of time, and compromise, relying mainly on kibble, with added scraps, and an occasional home-cooked meal as a treat. Scraps are fine so long as they fit into the dog's diet, and don’t involve sugar, or amounts of very fatty or salty food. Remember too that some human foods are harmful to dogs.

What foods are harmful to dogs?

Dogs shouldn't eat all human foods, though being scavengers, they will often try to. There are some human foods that dogs' innards can't cope with, and others that are actually poisonous to dogs, though not to humans. Corn cobs can kill dogs. It's safer not to eat corn on the cob if you have dogs, unless you can be absolutely certain that everyone disposes of the cob where a dog cannot get at it.

Chocolate designed for humans can overstimulate and even kill dogs. Grapes and raisins can also poison dogs, as can avocados, macadamia nuts, and castor oil seedcake or seed. Other foods to avoid include all products containing sugar, garlic, onions, tofu (which tends to produce gas), and anything with a lot of salt, including pub snacks designed for humans. This means that dogs shouldn't eat leftover human meals with a lot of onions, garlic, salt, or sugar.

A lot of problems with teeth and digestion come from treats and snacks rather than the main meal. Pub goers often like to settle their dog in a corner with a bag of snacks. Bagged pub snacks may have a high salt and content, may contain sugar, often include colouring or flavour enhancers, and tend to stick to the teeth. All this is very bad for dogs' teeth, and is not much good for their health in general. You can take a little bag of dog snacks to the pub, even a handful of complete dry dog food, rather than buying human snacks. It's good for dogs to socialise with pub people, but give these well-wishers your own dog treats to feed your dog.

What foods should dogs only eat in moderation?

Any food with a high fat content, such as fat meat, may be gobbled very greedily, but can then lead to a bad case of the squits, or trigger a serious illness. Fatty meat has to be rationed, and continual feeding of fatty meat is likely to cause nutritional imbalance and can cause serious illness.

Likewise, continually feeding dogs on salted meat doesn’t do them any good, though the occasional scrap of bacon is unlikely to do harm. Dogs with heart trouble are often prescribed low sodium diets. There is some debate as to how effective this is, but it's certainly safer to avoid giving anything high in salt to a dog with heart problems. Dogs with kidney trouble may benefit from diets low in phosphorous, though the calcium-phosphous ratio may be important rather than actual levels of phosphorous. Diabetic dogs shouldn’t be fed high levels of fat, or simple carbohydrates - which are found in some commercial dog treats. You can check with your vet which dried complete foods are safe for diabetic dogs.

There is some evidence that dogs that eat a lot of red meat, including beef and pork, are more likely to suffer from cancer, and possibly behavioural problems such as compulsive licking. Chicken is cheap, as are some oily fish, so it’s quite easy to vary protein sources.

Check the ingredients of dog treats. Usually they aren’t designed for more than occasional use, so if you use titbits a lot, for example, for training, try replacing treats with dry complete food, or make your own treats, or give rewards other than treats.

So what food is 'safe'?

Probably no food is totally safe for all dogs. Dogs can develop allergies to just about any food. You can feed the same diet to two dogs and find it leaves one with constipation and the other with loose bowels, so what’s safe depends a lot on your dog. However, most dogs can thrive on a variety of foods. If you home-cook, you need to provide variety for a balanced diet. Introduce new foods gradually, yes, but make sure there is a range of foods over the week. You could draw up a chart to make it easier to plan meals, with tripe twice a week, and oily fish at least once a week, for example. Offal, such as heart and liver, is also a suitable ingredient for home-cooked meals. Dogs do need calcium, and you can grind bones in a mincer, or buy packs of frozen minced meat and bones designed for pets. If you feed supermarket mince, you can add ground eggshells, using a coffee bean grinder to grind them, or they can be microwaved and then crushed. Minced meats are a useful standby, though they don't have teeth cleaning benefits of bones.

Dogs also benefit from vegetables added to their diet. Useful vegetable include those from the marrow family, like squashes and pumpkins, as well as root vegetables such as swede. Dogs often enjoy chomping on raw carrots, though they can also eat them cooked. Vegetables can be steamed lightly, then mashed in a food processor. Vegetables can provide fibre, which helps to prevent constipation. A little olive or sunflower oil also helps if you have a dog who cannot 'go'.

Should home-prepared dog food be raw or cooked?

Many people believe that dogs should eat raw meat, and claim to have seen the benefits in their dogs. Others believe that feeding raw meat involves too many risks, of malnutrition, infection, and damage from swallowing bones. There’s little hard evidence, partly because it’s difficult to design research projects on benefits and risks, when the term ‘raw feeding’ can cover many different practices, and some owners are more sensible than others. Some people feed raw meat and bones only, others add cooked and raw vegetables, and then there are people who mix raw and kibble, or simply provide an occasional piece of raw meat, like a turkey neck. As with home-prepared diets in general, the opportunities for making serious mistakes are greater when you feed raw. You need to provide a balanced diet, be especially careful about hygiene, and know what is safe for your particular dog.

All home-prepared food, whether cooked, raw, or mixed, should provide a balanced diet. Specialists with qualifications in canine nutrition are the best sources of information on recipes, even though some dogs seem to do well on DIY diets. Dogs are natural scavengers, and most can thrive on a wide range of foodstuffs. However, they can suffer malnutrition with some raw diets, for example, from calcium and other deficiencies from eating meat alone.

Calcium deficiency from meat-based diets has been linked to a higher risk of fracture in greyhounds. You can grind bones in a mincer if your dog has weak or no teeth, or you can buy packs of frozen minced meat and bones. As with cooked meals, eggshells are a useful source of calcium, the amount depending on the size of the dog. However, if you give your dog added calcium, especially calcium tablets, without expert advice, you could accidentally give your dog an overdose - excessive calcium supplementation has been linked to hip dysplasia.

Some owners feed just meat and bones, arguing that dogs should eat the way wolves do. However, dogs have adapted from living with humans to be able to digest starch, so, while animal protein is more easily digestible than grain or other vegetable protein, some vegetables are beneficial.

There is more risk of infection with fresh raw meat than freshly cooked. True, some dogs appear to have iron-clad stomachs, and eat rotten carcasses they find when on walks, then happily get rid of them by vomiting if the meal proves to be disagreeable. However, dogs can and do fall ill from infections caught from eating raw meat, and they can also pass on some infections to humans. It’s sensible to observe the commonsense precautions we take with our own food, such as using good quality, fresh meat, cooking or freezing the same day as purchase, and washing hands after handling raw meat.

Supermarket meat from intensively raised animals should be cooked. Intensive farming methods mean that meat is more likely to be contaminated by bacteria that could produce toxins, and harm both humans and dogs. Cooking won't get rid of the toxins, but it can kill the bacteria before they get a chance to multiply and create large amounts of toxin. If you plan on keeping any meat longer than 24 hours and can’t freeze it, it’s safer to cook. Chicken is especially likely to be contaminated by bacteria that could cause illness. Dipping meat in boiling water can reduce the risk of infection, because the surface tends to be more contaminated. Freezing meat can also kill some germs.

Your best bet if you want to feed raw meat is to make friends with a local butcher who cares about quality, and uses local sources, avoiding intensively-reared animals. Buy meat there for yourself, and ask for scraps for your dog. Then both you and your dog benefit. A lot that is discarded by butchers is in fact fit for human consumption, but we’ve become fussier, and often want no-waste meat, like steaks. Luckily, dogs benefit from eating our leftovers. A dog fed just on steaks may suffer both malnutrition and constipation. Raw cartilage, gristle and sinew provide bulk for the dog's intestine to work properly, and food that the dog doesn’t digest can provide a protective wrap round bones.

Wolves eat the skin and bones of rabbits, which give additional nutrients and fibre. Many rabbiting dogs will consume their catch raw, but not all dogs are designed as well as wolves for chewing up and digesting rabbits. Your dog may have a small jaw and small teeth, or have lost some teeth. Some owners mince raw food for older dogs with problem teeth, so the dog at least gets the benefits of nutrients that might otherwise be lost in cooking.

Be cautious about feeding raw food to pups or very old dogs, because they have weaker immune systems. Dogs with optimum immune function are unlikely to suffer from germs like salmonella which can contaminate raw meat, but older dogs and pups are more vulnerable. Neosporosis cranium is another risk, though it is rare. It’s a nasty disease which generally only affects puppies and is usually passed to the pups from their mother. Raw food can be a source of this infection, but it’s destroyed by both cooking and freezing. Freezing and then thawing raw meat is one way to ensure that it is not contaminated (and as with human food, the food should be consumed or junked once it has been thawed, and should not be refrozen).

A great advantage of giving dogs raw bones to chew on is that it can strengthen their teeth, a major benefit reported by owners who have switched to raw feeding. However, owners and vets also report that the wrong sorts of bones can wear down teeth. Some bones can also be dangerous for dogs that tend to swallow food without chewing (often the case with Labradors). If you feed raw bones, it's important that they’re suitable bones for the dog in question. Dogs that have been too aesthetically altered from the original blueprint (like many Pekes, for example) may not be able to chew on bones at all, especially if the dogs are elderly.

There are two possible behavioural downsides to raw feeding. One is that dogs often value raw bones very highly, and may try to defend them more strongly than they would a boring bowl of kibble. People with more than one dog need give each dog enough space to chew in peace, and make sure no 'ownerless' bones lie around to provoke squabbles.

Raw feeding has become very popular, so there are many internet groups that you can join to learn more. These groups can often provide a lot of practical help, such as lists of reliable suppliers, and pointers as to which types of bones tend to be safe and which are more problematic. They tend to be less useful in helping novices design a balanced diet, unless there are qualified nutritionists among the contributors. A sympathetic vet can be a great ally in pointing out possible risks and benefits of feeding raw, and a vet who has also studied nutrition in depth is an invaluable ally.

Can dogs eat cat food?

Cats and dogs have different nutritional needs, so it's not a good idea to feed cat food to dogs. Dogs living with cats may steal the cat's food. Try putting the cat food where the dog can't reach it, or supervise feeding sessions and remove the bowls when the animals have stopped eating.

Should puppies be given cow's milk?

Some pups and dogs are intolerant of cows' milk, which can give them diarrhoea. If this happens, stop feeding cow's milk. There are special formulations which you can buy for hand rearing pups, or for helping out their mum if she has a lot of pups and is showing signs of wear and tear. These formulations can be very expensive when bought from a vet in Britain, so try buying them online.

What about grass?

Grass is a natural emetic, and you may have noticed your dog eating grass to be sick. This is normal, though give your dog a chance to vomit outside if he eats a lot of grass, so he's not sick indoors. Some dogs seem to be especially fond of grass, and you may need to put your houseplants out of reach, because they are tempting if no grass is available. It's not fully understood why dogs eat grass, though one reason could be a need for roughage. If you can, keep a safe patch of longish 'dog grass' in your garden, so your dog doesn't resort to your precious plants. Some garden and house plants can also poison dogs, such as euphorbias and plants from the lily family. Growing ´dog grass´ also allows your dogs to eat grass that hasn't been sprayed with pesticides or urinated on by other dogs.

Can changes in diet help dogs with medical disorders?

Yes, very much so, but it's worth checking with your vet before changing your dog's diet, and it’s essential to do so if your dog has a serious illness. Boiled egg and rice is a classic remedy for squitty dogs. Let the dog fast for a while with access to water, then feed him small meals of egg and rice. Some dogs also tolerate boiled chicken and rice when they are squitty. These are not good permanent diets, since they are not nutritionally complete.

Roughage is important for dogs with anal gland problems, because it helps them empty their anal sacs. This is one reason why meat with sinew, cartilage and other indigestible materials is better than steaks alone, though small amounts of bran or brown rice may do the trick. Roughage and having several small meals a day may also help dogs with diabetes, while diabetic dogs tend to do better on complex carbohydrates than simple carbohydrates.

Fish oil appears to be beneficial for dogs with a number of conditions, such as skin problems, wound inflammations, cancer, heart disease, and arthritis. Dogs with heart disease, kidney problems and diabetes benefit from special diets, and may benefit from supplements, for example, carnitine and taurine for dogs with heart complaints.

Dogs with food allergies may benefit from a move to a diet based on lamb, rabbit, chicken and rice, which they are less likely to be allergic to, excluding beef, cow's milk and cereal, which they are more likely to be allergic to. Elimination diets are often used to assess whether certain foods might be causing problems. Dogs are started off on a small range of foods which are seen as least likely to trigger allergies, with more suspect foods gradually added, one by one, to see whether they trigger the complaint. You may need to change your dog's diet for as long as 10 weeks to see an improvement, though usually there's some improvement after six weeks or so.

There are references to research articles on diet and medical disorders in Further Reading, at the end of this article. It’s safer to discuss information you find on nutrition and any illness your dog may have with your vet rather than trying DIY remedies yourself, otherwise you could end up making your dog sicker, with the best of intentions.

Does diet affect dogs' teeth and gums?

Yes, a lot. Sugar is of course bad for teeth, both for humans' teeth, and for dogs'. Calcium deficiency, eg from an all-meat diet, weakens teeth and bones in general. Hard and chewy foods are better for gums and teeth. Dogs are less prone to tartar, stained teeth and gum disease if they have chews, though many owners prefer raw bones to keep their dogs' teeth clean. Dogs have been known to swallow chews, both of the hide and knotted type, which can cause serious problems. Raw vegetables, such as carrot, can also help to keep teeth clean, though the size of the pieces will depend on the dog. Some dogs chew carrot, others swallow large items whole, so large pieces of raw vegetables are not safe. Corn cobs are not safe for any dog.

‘Extras’ like tidbits, tend to cause more dental problems than kibble alone. Some special dog titbits contain colourings that can stain teeth. Titbits may also contain sugar, salt and a lot of fat. Dogs constantly fed on titbits are more likely to have dirty mouths that bacteria can thrive in, so it you use titbits for training, try not to feed them throughout the day. Titbits designed for humans, like sweet biscuits or pub snacks, are especially bad for teeth. Dogs that are highly motivated by food will usually work for kibble. It’s worth thinking about what a dog wants at any moment, and using that as a reward, rather than automatically feeding a treat; for example, if a dog really wants to go out, you can insist on a sit before opening the front door, and opening the door is a bigger reward than food.

Inflammation can be triggered by dogs having dirty mouths, but is also affected by stress and poor diet in general. Vitamin E and omega-3 fatty acids may help control gum disease by reducing inflammation. Tooth brushing also helps to keep teeth clean. However, it’s more efficient to prevent dental problems by providing something to chew on, giving the dog a balanced diet, and restricting titbits.

Will low-protein foods make my dog behave better?

The short answer is probably no, and certainly not on their own. Ten minutes formal training a day, and a good long walk, are likely to be beneficial whatever changes you make to your dog's diet. A bored dog who is rarely walked is likely to be difficult to handle whatever he eats.

One study suggests that low-protein diets do not affect 'dominance aggression', or 'hyperactivity', but can reduce territorial aggression linked to fear. Later research suggested that the effect of reducing protein levels might be be greater if tryptophan supplements were also used. However, putting a lot of effort into training your dog may be a better way of reducing territorial aggression. There are also risks involved in cutting back on protein. Pups need higher protein levels than adult dogs. Low protein diets have been cited as a risk factor for hip dysplasia. They have also been linked to anaemia in the case of working sled dogs, which need stamina for covering large distances, and which do better on a high protein diet, so check with your vet before making changes.

There are other aspects of diet that may affect behaviour, for example, the protein source may be important, rather than actual levels of protein. Certain colourants have also been linked to behavioural problems.

How much food should I feed my dog?

Again, there’s no simple answer to this question. Dogs vary a lot in terms of how fast they burn up food. Very active dogs such as working dogs, need to eat more, as do pregnant and lactating bitches. Lactating bitches may need two to four times as much food as usual - this depends a lot on how many hungry pups they have to feed, and on the breed. Young dogs may go through growth spurts and dormant phases, so their needs and appetites can vary a lot over a month. Senior dogs need fewer calories, but more of some minerals and vitamins, and food that is easier to digest, which is why some manufacturers offer them special formulations. Sled dogs, which spend a long time in vigorous exercise, do well on high-fat, high protein diets, whereas greyhounds, which just run for short periods, do better on high-fat and moderate protein diets. Sled dogs eat more than sedentary dogs, and may need fewer vitamins and minerals per unit of energy intake (joule). Unfortunately, dog food packs and cans tend not to give information on calories per serving or per unit of weight. As a rough guide, if a dog is fit but underweight, feed more food, but if the dog is sluggish, the nutritional content of the food may need to be improved.

Dog nutrition books provide tables for dogs of different weights and lifestyles, as well as recipes for homecooked meals. If you use commercial foods, go by manufacturers' recommendations, and try to select a formulation for your dog's type, age and lifestyle. The amounts recommended are just guidelines, though, not rigid rules, and anecdotal evidence suggests that they may be too generous for the average dog. If your dog is also fed tidbits and scraps, these need to be included in calculations.

Adjust amounts if your dog leaves food in his bowl, or appears to be getting too skinny or too fat. Your dog should have a waist, and you should be able to feel the ribs. If not, the dog is probably too fat. If the backbone feels very bony and knobbly, the dog is probably underweight. Fur affects how dogs appear - dogs with thick underfur tend to appear fatter than they are. Owners shouldn’t worry greatly about dogs that aren’t eating, or aren’t eating as much as usual, so long as the dog isn’t underweight and seems otherwise fit. Gravy made from boiling a few bones with nothing else added, can help to stimulate a poor appetite. Timing is important – dogs are often hungrier at particulat times of the day. Sometimes dogs just aren’t hungry because it’s too hot, or they fancy the bitch down the road. A missed meal or two won’t harm a healthy dog, and may even be beneficial.

Obesity is generally a more serious problem in the UK than underfed dogs. Obesity is linked to many health problems, like diabetes, arthritis, mammary cancer, and incontinence in bitches. Rapid growth in giant dogs is linked to bone problems. Obese dogs suffer more wear and tear on their joints if they’re arthritic. Obese dogs are often fed no more at mealtimes than normal dogs, but get lots of titbits. So, if your dog is a bit on the podgy side, give him or her ball games, cuddles, or anything else he or she likes that isn't food, as a reward, and of course more exercise. Spayed bitches can pile on the kilos, so go by the look of your dog, rather than worrying about how little she eats, and get a vet to check her if you worry she may be too fat.

How often should my dog eat?

Puppies need to eat several times a day, while adult dogs may eat once or twice a day. Some dogs seem happy on one meal a day, but twice a day is safer, especially in the cases of active and/or large-bred dogs. Dogs only fed once a day may be more at risk from digestive problems if they’re very active shortly after their meal. GDV is a particular problem with large breeds, and can be fatal. Give your dog at least an hour to digest his food before taking him out for a run. Most dogs should have their main meal after exercise, rather than before, and should have some time to relax after they come in from exercise, rather than being fed as soon as they come in. Diabetic dogs may, however, benefit from exercise after a meal, and they need several short walks rather than one longer bout of exercise.

You can replace a meal with complete dry food, and use this as titbits for training a young dog out on walks, for example to reward him for coming back. If you do this, make it a small meal, so as not to overload his stomach while he's running about.

Free-feeding isn’t a good idea. It can encourage obesity, food exposed to the air for a long time is likely to lose nutritional value and become contaminated. If you have more than one dog, free-feeding makes it difficult to assess how much an individual dogs is eating, and dogs may fight when allowed unsupervised access to food. It's best to try to feed dogs at set times. This is especially important when you are housetraining puppies, so you can get into a routine of when they ‘perform’.

Can I give my dog a vegetarian diet?

This isn’t advisable. Though it can be beneficial for dogs to eat some vegetables, they have more need for meat and fish than humans do, and it’s very difficult to feed them properly without giving them at least fish and eggs. Working dogs on low protein vegetarian diets may be more vulnerable to anaemia than if they are fed animal-based low protein diets.

What can research on dogs and diet tell us?

Many people complain that most research on canine nutrition is funded by pet food companies. Check out some of the articles written by researchers in the industry and see for yourself. Pet food companies will tend to fund what's in their interests to research, for example, what to put in their products, but you can use their findings for your home-cooked recipes. Remember, though, that studies may not look at the long-term effects of feeding a particular foodstuff, just whether or not dogs seem OK with it for a few weeks or months.

There has also been a lot of research into sled dog and racing greyhound diets. While this is interesting to read, remember that these are very active dogs, and it’s important to devise a diet that’s appropriate for your dog’s lifestyle. If your dog just has two short walks a day, it’s not sensible to give your dog a diet suitable for a working husky.

It's very difficult to 'prove' anything in science. Research can tell us about possible links, say between red meat and cancer, or whether a particular additive seems safe, but results can be affected by several factors. One is how long the study lasted for. Unless long-term studies are carried out, you can’t tell the long-term effects of feeding an ingredient. Another factor is the sample size (eg how many dogs were involved, since small groups may be atypical). The categories used, and the way they are defined is also important, for example if all ‘raw fed dogs’ are lumped together this can be misleading, since raw feeding practices vary so much. The amount given of any ingredient is important, because some ingredients may be poisonous in excess, but necessary in small amounts.

There may also be variables you haven’t thought of that have affected the study, for example, it's difficult to separate the effect of diet on behaviour from other factors, like how owners treat and perceive their dogs. The breed may also be significant in studies of nutrition and health, since some breeds are more prone to certain illnesses than others.

Do talk to your vet, or a canine nutritionist about any research you’ve read, because you may gain a better understanding of what the research means. Your vet can also advise you on amounts, for example of roughage, or fish oil supplements, that are appropriate for your dog

Acknowledgements: Thank you to Diana Attwood, Shelly Couvrette, Amy Dahl and Wendy Hanson for informative discussions on dogs and diet, and to Diana for contributions to this article.

Laflamme D.P. Nutrition for aging cats and dogs and the importance of body condition. Vet Clin North Am Small Anim Pract. 2005 May;35(3):713-42.

Lee, K.W. et al Hemotologic changes associated with the appearance of eccentrocytes after intragastric administration of garlic to dogs. American Journal of Veterinary Research 2000 Nov; 61(11): 1146-1150